Flow Anisotropy due to Thread-Like Nanoparticle Agglomerations in Dilute Ferrofluids (original) (raw)
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Magnetoviscosity and thread-like agglomerations in ferrofluids
2011
to achieve sufficient spatial and temporal resolution to track the dynamics of these 500µm diameter spheres simultaneously with an array of magnetic particle macro-chains-thread-like agglomerations each several mm long and 2 − 10µm thick. The enhanced drag induced by the macro-chains is enormous: up to four times larger than for unmagnetized fluid, a value greater than is predicted by the prevailing magneto-viscosity model. We provide direct visualization of a possible mechanism by which macro-chains impede the transverse motion of spheres. Numerical simulations can reproduce the observed drag, without modeling it physically, by implementing a simple magnetization dependent anisotropic viscosity.
Journal of Magnetism and Magnetic Materials, 2011
Rheological experiments on relaxation of shear stress in a diluted ferrofluid with clustered iron oxide nanoparticles (f ¼ 0:1 vol%) have been performed. Changes of the stress appearing in the fluid at constant magnetic field strength after a stepwise change of shear rate g Á have been measured. It has been found that the time of the transient, until the shear stress in the fluid will be steady, depends strongly on the strength of the applied magnetic field and shear rate. For vanishing magnetic field slow relaxation has not been observed. The time of the transient in the presence of a magnetic field can reach several minutes. The change of the transient viscosity Z t ¼ 1=g Á ðg Á Þ as a function of the steady viscosity Z t-1 ðg Á Þ shows a linear behavior and depends on magnetic field strength. Those effects can be attributed to the process of structure formation and destruction due to the simultaneous action of an applied magnetic field and shear flow. A similar behavior is known from the rheology of complex fluids like polymer melts or aggregating colloids. We propose a model of the rheological effects based on the assumption that the clusters form linear chains which size distribution is determined by applied magnetic field and shear rate.
Magnetorheological properties of ferrofluids containing clustered particles
Colloid Journal, 2013
A theoretical model is proposed to describe experimental data on the magnetorheological prop erties of magnetic fluids containing clustered particles consisting of single domain ferromagnetic nanoparti cles distributed in a polymeric shell 80-100 nm in diameter. These fluids combine the sedimentation stability typical of nanodisperse ferrofluids with the high sensitivity of rheological parameters to magnetic fields. The developed model explains the experimentally found long term rheological relaxation and residual stress that is retained after the medium ceases to flow.
Physical review fluids, 2023
We present a three-dimensional computational study of the impact of external magnetic fields on the dynamics of superparamagnetic ferrofluid droplets and rheology of dilute ferrofluid emulsions in planar extensional flows. Specifically, we show how the intensity and direction of uniform magnetic fields affect the planar extensional rheology of ferrofluid emulsions by changing the shape and magnetization of the constituent ferrofluid droplets in suspension. We find that the two traditional extensional viscosities associated with the normal stresses of the bulk emulsion in extension either remain constant or increase with the field intensity; the only exception occurs when the field direction is perpendicular to the extension plane, where increasing the field intensity keeps the planar extensional viscosity constant and modestly decreases the second extensional viscosity. We also find that the droplet tilts in the flow when the external field is not aligned with one of the flow main directions, which changes the recirculation pattern and flow topology inside the droplet. At the microscopic level, the droplet experiences a magnetic torque because of a small misalignment between its magnetization and the external field direction. At the macroscopic level, the bulk emulsion experiences a field-induced internal torque that leads to a nonsymmetric stress tensor with unexpected shear components in extension. To account for this unconventional stress-strain response, we introduce new extensional material functions such as shear and rotational viscosity coefficients that unveil novel rheological signatures of ferrofluid emulsions in planar extensional flows. This study offers new insights into applications based on the field-assisted manipulation of ferrofluid droplets and sheds light on the potential of ferrofluid emulsions as a model system for chiral fluids with internal rotational degrees of freedom that can be activated and controlled by coupling static magnetic fields with hydrodynamic flows.
Study of Aqueous Dispersions of Magnetic Nanoparticles by Magnetic and Rheological Measurements
2016
The observed magnetic tunability of light transmission through a ferrofluid can be effectively understood in terms of the inter-particle interaction that can be estimated from the magnetic and rheological properties of these fluids. The present study reports complementary magnetic and rheological measurements of aqueous dispersions of ferrite nanoparticles and a commercial ferrofluid. The room temperature magnetization measured in a SQUID magnetometer up to fields of 1 to 2 Tesla showed superparamagnetic behaviour of the particles and the dispersion with the background signal of the liquid showing a diamagnetic behaviour. The room temperature rheological behaviour in zero magnetic field of the fluids was investigated by measuring the viscosity as a function of shear rate from 1-100 s-1. The particle size and the nature of the carrier liquid determine the viscosity and is expected to have an effect on the inter-particle interaction.
Journal of Colloid and Interface Science, 2000
Static magnetization curves and the magnetorheological effect were used to study the microstructural properties (agglomerate formation) of magnetic fluids and the properties of dispersed nanoparticles. Improved techniques for magnetogranulometry analysis and a formula for the magnetoviscous effect were proposed. The area of applicability of some existing models was studied. The density, distribution, and dimension of particles, as well as the thickness of the nonmagnetic layer were accurately determined from magnetic measurements. The Shliomis diameter and the effective anisotropy constant were determined from rheological and magnetorheological measurements using information obtained from magnetization curves.
Journal of Engineering Mathematics, 2017
Ferrofluids are typically suspensions of magnetite nanoparticles, and behave as a homogeneous continuum. The production of nanoparticles with a narrow size distribution and the achievement of colloidal stability are important technological issues. The ability of the ferrofluid to respond to an external magnetic field in a controllable manner has made it emerge as a smart material in a variety of applications, such as seals, lubricants, electronics cooling, shock absorbers and adaptive optics. Magnetic nanoparticle suspensions have also gained attraction recently in a range of biomedical applications, such as cell separation, hyperthermia, MRI, drug targeting and cancer diagnosis. In this review, we provide an introduction to mathematical modeling of three problems: motion of superparamagnetic nanoparticles in magnetic drug targeting, the motion of a ferrofluid drop consisting of chemically bound nanoparticles without a carrier fluid, and the breakage of a thin film of a ferrofluid.
Field-dependent anisotropic microrheological and microstructural properties of dilute ferrofluids
The European physical journal. E, Soft matter, 2014
We have measured microrheological and microstructural properties of a superparamagnetic ferrofluid made of Mn0.75Zn0.25Fe2O4 (MZF) nanoparticles, using passive microrheology in a home-built inverted microscope. Thermal motion of a probe microsphere was measured for different values of an applied external magnetic field and analysed. The analysis shows anisotropy in magneto-viscous effect. Additional microrheological properties, such as storage modulus and loss modulus and their transition are also seen. We have also obtained microstructural properties such as elongational flow coefficient [Formula: see text] , relaxation time constant [Formula: see text] , coefficient of dissipative magnetization [Formula: see text] , etc., using the analysis given in Oliver Muller et al., J. Phys.: Condens. Matter 18, S2623, (2006) and Stefan Mahle et al., Phys. Rev. E 77, 016305 (2008) over our measured viscosity data. Our values for the above parameters are in agreement with earlier theoretical c...